(1) Field of the Invention
The invention relates to integrated MOSFET semiconductor circuits, and more in particular to a translinear amplifier. According to the objectives of the invention said translinear amplifier is intended lo work linearly within a wide specified region and to achieve sharp cutoff edges at the limits of said linear operating region, to force e.g. a switching transistor, which said translinear amplifier may be intended to drive, into minimum RDSon or maximum RDSoff. A possible, but not exclusive, use is to control a chain of switching transistors for a voltage controlled capacitor, formed by a larger number of small capacitors.
(2) Description of the Prior Art
In a translinear amplifier, the output voltage difference is kept at the same relative difference as the input voltage difference, independent of the absolute output voltage level. Said translinear amplifier is normally formed by two current balancing circuits and some form of an amplifier stage, where the amplifier forces the output voltage difference to be exactly the input voltage difference.
FIG. 1 (Prior Art) shows the block presentation of a translinear amplifier and FIG. 2 (Prior Art) shows the principal components diagram of a translinear amplifier.
In FIG. 2 (Prior Art) a first current dividing circuit with two branches, N1 and N2, and with a common current source I1, is connected to said translinear amplifier's inputs Vin-p and Vin-n. A second current dividing circuit with two branches, N5 and N6, and with a common current source I2 is connected with said translinear amplifier's reference voltage Vr f and with said translinear amplifiers output Vout. In both current-dividing circuits, a current probing circuit (N4 and N8) determines the current in one of its branches, which is a measure of the current ratio within each said current dividing circuit. An amplifier AMP compares both current probing signals and, in a closed feedback loop, forces the output voltage Vout to change until both values measured at said current probes are the same. The amplifier AMP generates Vout-p such that I2p=I1p, which is only true if Vin-p−Vin-n=Vout-p−Vout-n. The absolute voltage level of Vout p and Vout n is independent of the level of Vin-p and Vin-n. As can be seen, the translinear amplifier works as a perfect level shifter with a gain of 1. The translinear amplifier operates linear in a wide voltage range, however the characteristic is more or less undefined outside this normal operating range, causing soft cutoff edges at the limits of said linear operating region.
U.S. Pat. No. 6,121,824 (to Opris) describes a translinear circuit with series resistance compensation, where a sub circuit including three bipolar transistors is substituted in place of a single bipolar transistor to achieve extrinsic base and emitter series resistance compensation in translinear circuits.
U.S. Pat. No. 5,432,474 (to Lauffenburger and Moore) discloses a fixed and adjustable bandwidth translinear input amplifier. An improved method and circuitry for processing high-impedance (current mode) input signals for use in translinear and other mode circuits in a manner that avoids the signal dependent bandwidth variations that occur in the prior art. A non-linear feedback structure using a transconductance gain element is employed to extend the bandwidth and/or suppress bandwidth variations.